XP still has an edge as most of the current games were designed to run on it, using DirectX 9, and nVidia, ATI, and the others had years to tweak the drivers for maximum performance. None of that is the case with Vista, so most are finding XP still gives gamers the edge. Thus, if you are looking to give him the best gaming performance, XP is still the way to go.
As to x64 vs x86, driver and security software compatibility are the follies of 64-bit computing. Actual performance can vary, just as when comparing single core processors to dual core models, neither claiming a unanimous victory. The key benefit to 64-bit versions of Windows is the elimination of the 4GB barrier when it comes to RAM, but that shouldn't apply in your case since you're only installing to 2GB. Thus, the choice is yours, though I would take x86 to avoid potential driver issues.
Hope this helps,
I am currently building a gaming computer for a friend of mine, and am a little stumped trying to decide which operating system I should install for him. I haven't used Vista myself and thus would like to get the opinions of other users. Is Vista a solid platform for gaming, or should I install XP on his machine? And is there any reason to go for the 64-bit reason versus the 32-bit version? The only downside I see to the 64-bit version is driver support.
And if anyone is interested, here are the specs of the PC I hope to build for him:
Intel Core 2 Duo E4300 1.8GHz
MSI 945GM3-F Mobo
2x512MB Kingston ValueRAM DDR2 800
Seagate Barracuda ES 320GB SATA HD
EVGA GeForce 7600 256MB PCI Express x16
Creative SB Audigy SE 7.1
Sony NEC 18x DVD/CD Burner